A commercial drive exists for reducing power consumption in electronic devices. In support of this effort, industry has attempted to exploit digital signal processing techniques to minimize the usage of high power consuming analog componentry. Digital processing necessitates the conversion of continuous analog signals into a digital data format using an analog-to-digital converter ("ADC").
ADCs convert analog signals into discrete digital data by performing a series of functional steps. These process steps include sampling, holding, quantizing and encoding. Though unique, these four steps need not be performed as independent operations. It is known, for example, to perform the sample and hold functions simultaneously using a single circuit.
Referring to FIGS. 1(a) and 1(b), a known circuit 10 for sampling and holding an analog signal, V.sub.IN, is shown. Sample and hold circuit 10 comprises a metal oxide semiconductor ("MOS") type transistor M.sub.a having a source for receiving the continuous analog signal, V.sub.IN. Further, the gate of transistor M.sub.a receives a sample signal, .PHI..sub.a, which comprises a series of pulses. Each pulse of sample signal, .PHI..sub.a, has a width, .tau., and a sampling periodicity T.sub.s. As illustrated in FIG. 1(b), at the intervals when a sampling pulse of sample signal, .PHI..sub.a, is received by the gate of transistor M.sub.a, a segmented portion corresponding with the pulse width, .tau., of the pulse and the relative height of the continuous analog signal, V.sub.IN, is captured as a sample. Thereafter, the sample is transferred to a capacitor, C.sub.a, for interim storage. The held samples are represented by V.sub.OUT.
One problem with ADCs, particularly when realized in MOS technology, is the linearity of the impedance of the sampling switches. As in the circuit of FIG. 1(a) hereinabove, the MOS transistor M.sub.a is turned on and off by the sample signal, .PHI..sub.a, to produce the samples found in V.sub.OUT. However, a relationship exists between the inherent impedance of the switch of circuit 10 and the input signal, V.sub.IN. Upon receiving a sampling pulse of sample signal, .PHI..sub.a, the impedance of the switch of circuit 10 is a function of the difference between the gate to source voltage ("V.sub.GS ") of transistor M.sub.a and the threshold voltage ("V.sub.TH ") of transistor M.sub.a for the duration of pulse width, .tau.. The impedance of the switch of circuit 10, while a sampling pulse is received by transistor M.sub.a, is also referred to as R.sub.ON, and may be mathematically represented by the following formula: ##EQU1##
where .mu..sub.n is the electron mobility, C.sub.ox is the capacitance of the gate oxide, W is the width and L is the length of the channel of transistor M.sub.a, assuming the drain to source voltage ("V.sub.DS ") of transistor M.sub.a to be inconsequential and the applicability of square law behavior. As may be viewed by the above mathematical expression in view of circuit 10 of FIG. 1(a), V.sub.GS is equal to the difference between the "on" peak voltage of the pulse of sample signal .PHI..sub.1, or V.sub.DD, and the input signal V.sub.IN. PA1 where V.sub.THo is an initial threshold voltage constant, .gamma. is a body effect parameter and .PHI..sub.f is a quasi-Fermi potential of transistor M.sub.a.
Moreover, V.sub.TH also functionally corresponds with V.sub.IN by means of source bulk voltage ("V.sub.SB "). V.sub.TH may be mathematically represented by the following formula: EQU .sub.TH =V.sub.THo +.gamma.*[2.vertline..PHI..sub.f.vertline.+V.sub.SB +L -2.vertline..PHI..sub.f +L .vertline.]
Given the hereinabove mathematical expressions, the "on" resistance, R.sub.ON, is therefore a non-linear function of input signal V.sub.IN. Signal distortion is a natural byproduct of the mathematical relationship of R.sub.ON with input signal V.sub.IN, generally, and more particularly if the voltage levels of V.sub.IN change rapidly. Thus, efforts to lower signal distortion have focused on reducing the value of R.sub.ON, as well as its dependence on the input signal V.sub.IN.
Several solutions have been proposed to reduce the dependence of R.sub.ON on input signal V.sub.IN. Each of these approaches, however, have particular shortcomings. These limitations include raising additional non-linearities, as well as failing to eliminate the dependent relationship between V.sub.TH on input signal V.sub.IN and thus R.sub.ON with input signal V.sub.IN, irrespective of whether the frequency of the sample signal is or is not much greater than V.sub.IN.
As such, there is a need to provide a sampling device having a switch with a gate source voltage, and, thus, an "on" impedance, R.sub.ON, that is independent of the input signal being sampled. Likewise, there also exists a demand for a sampling device having a switch with a threshold voltage, and, hence, an "on" impedance, R.sub.ON, independent of the input signal being sampled. Moreover, there is a need for a sampling device with a switch having an "on" impedance, R.sub.ON, independent of the input signal being sampled which does not raise additional non-linearities, irrespective of whether the frequency of the sample signal is or is not much greater than that of the input signal.